Traditional cellular networks in use today rely on a thick-client approach for network edge signal processing for voice calls as well as for data calls. More particularly, for calls originating at land-lines, a Public Switched Telephone Network (PSTN) local office typically converts an optical signal to base band frequency (BBF) for delivery to the Mobile Switching Center (MSC). The MSC switches the signal to a Base Station Controller (BSC) that conveys it to one of several Base Stations (BTS) with radio frequency (RF) management instructions. The BTS then uses an antenna to down link this signal to the user. Calls originating from users are up linked to the antenna adjacent to a BTS. The BTS conveys the call upstream via the BSC and MSC into the PSTN.
When a call is processed from a downlink perspective, the BTS typically performs many functions. For instance, the BTS may interface with the landline voice circuits from the BSC. The BTS may also combine several voice circuits into an RF channel. More specifically, several base band circuits are combined and converted into the desired radio frequencies. As another example, the BTS may also amplify and transmit the RF channel to the remote users.
When the call is processed from an uplink perspective, the BTS may be configured to receive the RF channel that includes the signals of multiple remote users. The BTS may also separate voice circuits from the received RF channel. More specifically, the RF signal is signal processed through low-noise amplifiers, RF filters, and digital and/or analog baseband processing. Finally, the BTS may be configured to transport voice circuits to the BSC through analog modems. Since significant call processing is conducted at the edge of the network this approach may be called the thick client paradigm. An example of the thick client paradigm may be found in U.S. Pat. No. 4,901,307, which is hereby incorporated in its entirety by reference.
Although the thick client paradigm is a workable solution, it is not without its drawbacks and disadvantages. For example, during downlink and uplink processing, the BTS generally requires a large amount of processing capacity, which is typically supplied by relatively expensive digital signal processors (DSPs) and application specific integrated circuits. The DSPs are often application specific and produced in limited quantity. Moreover, these DSPs do not generally allow for cross applications such as WiMax, GSM, CDMA or other protocols. As a result, carriers are required to maintain expensive independent and parallel equipment and networks to support these other protocols.
Another approach in cellular call processing is the thin-client paradigm. An example of this thin-client paradigm is described in U.S. Pat. No. 5,627,879, (“the '879 patent”) which is hereby incorporated in its entirety by reference. In the thin client paradigm, the BTS is separated into two parts: an antenna unit (AU) and a base station unit (BSU). This thin client approach offers an advantage that, the AU is wireless protocol agnostic (WiMax, CDMA, GSM, etc.). However, a problem with this approach is that digitization of the RF signal increases the bandwidth requirements for the link between the AU and the BSU, and traditionally used T−1's, T−3s, or microwave links are often inadequate, requiring the use of optical fiber which is still not ubiquitously available.
In one embodiment described in the '879 patent, decimation filters are provided for filtering out unwanted channels from the digital stream so that the signals from the antenna units can be moved to and from base station units over lower speed carriers such as T−1 lines. While this embodiment solves a backhaul bandwidth problem it reintroduces two other problems. First, additional digital signal processing required at the AU ensures that it does not remain a thin client. Second, the AU is again protocol constrained as in the thick client approach.